The present invention relates to a motor driving device, and more particularly relates to a motor braking technique in a device for driving a brushless motor by a drive signal generated with one cycle delay from a rotation signal output from a rotational position detector such as a hall.
Methods for braking a brushless motor include reverse braking and short braking. Reverse braking is a method in which a current having an opposite polarity to a polarity in positive rotation is supplied to motor coils of different phases to excite the motor coils, thereby reducing the rotation speed of a rotor. In reverse braking, braking force strongly affects and thus the rotation speed of the rotor is rabidly reduced. However, as the rotation speed of the rotor is reduced, braking becomes unstable. Thus, there might be cases where the motor can not be stopped. On the other hand, short braking is a method in which impedances between motor coils of different phases are lowered and then the impedances are short circuited to a ground voltage or a power supply voltage to eliminate an induction voltage of each phase, thereby braking the motor. In short braking, braking force does not affect as strongly as in reverse braking but the motor can be reliably stopped. Therefore, in a brushless motor, reverse braking and short braking have to be appropriately switched around to perform effective braking.
There is a known motor driving device in which an electrical angle rotation period is calculated and, when the period exceeds a threshold, a brake mode is switched from a reverse brake mode to a short brake mode (see, for example, Japanese Laid-Open Publication No. 2003-235287). Accordingly, a motor can be stopped in a short time.
Moreover, as a device for driving a brushless motor or the like, there is a device in which detection intervals for a reference position of a rotor are measured while the rotor is rotating, a drive signal having an approximate sinusoidal waveform is generated based on a rotation period of the rotor, i.e., the electrical angle rotation period, and energization of phases of a motor is controlled according to the drive signal (see, for example, Japanese Laid-Open Publication No. 2004-187454).
In the technique used in the latter one of the above-described devices, the motor is driven by the drive signal having a non-deformed approximate sinusoidal waveform. Thus, excellent motor drive control characteristics at a constant speed can be achieved. However, in the motor driving device, when reverse brake is applied, a deformed waveform of an actual motor induction voltage does not match a non-deformed waveform of the motor drive signal in some part, so that speed acceleration control is performed in the part. Specifically, in the motor driving device, the drive signal is generated with one cycle delay from a rotation signal representing detected rotation of the motor, so that a percentage of the mismatch part between the motor induction voltage and the motor drive signal is increased where the speed of the motor is decelerated. This might cause a case where deceleration and acceleration of the motor are repeated and then the motor can not be stopped.
To cope with this, the technique used in the former one of the above-described devices is applied to the latter motor driving device so that motor braking is started in a reverse brake mode and, when the electrical angle rotation period of the motor exceeds a threshold, the brake mode is switched to a short brake mode. Thus, the motor might be reliably stopped in a shorter time. However, even immediately before the reverse brake mode is switched to the short brake mode, it is uncertain which the motor has been subjected to deceleration control or acceleration control. If the motor has been subjected to acceleration control, it takes a longer time to stop the motor after a switching timing to the short brake mode.